The present invention relates to heat treatment and processing of cast steel railroad wheels. More specifically, a method and associated apparatus are disclosed for increasing or reducing the rate of heat loss from as-cast railroad wheels.
Historically, railroad wheels have generally been produced by forging or casting of cast iron or cast steel. The cast steel wheels are primarily produced by a bottom-pouring casting technique. However, any of the production processes require control of the cooling cycles or rates to maintain the crystalline microstructure of the cast or forged wheels. In the above-noted bottom-pouring technique, the as-cast wheels are removed from the casting mold for transfer to subsequent operations to remove the hub core, sprues and risers, for inspection and for heat treating and normalizing at various production stages.
Although the railroad wheels are normalized, their microstructure is influenced by the as-cast temperature and the subsequent cooling rate. This is especially influenced by mass differences from the relatively thick cross-section at the outer tread portion, through the thinner connecting web, to the most massive section of the wheel at the axle hub. The cooling rate influences the microstructure, the rate of formation of inclusions in the grain boundaries, their distribution in the microstructure, dislocation formation, dislocation movement across the grain boundaries, residual stresses and their locations, as well as other metallurgical and mechanical properties. In addition, the cooling cycle must provide the wheel at the subsequent production operation from the shake-out at the correct temperature for the next operation, which is not necessarily room temperature.
Production practices have required transport of railway wheels on a continuous conveyor path through an in-line kiln. In general, the structure of the kiln provided refractory lined walls and roof in an elongated chamber similar to a muffle furnace. In at least one known operation, the capacity or rate of heat transfer could be accelerated by raising the upper or roof panels to provide a greater volume of air flow past the wheels. The cooling practice did not desire or require a water or hot oil quench, and the slower cooling rate from air cooling or air quench practice to achieve the desired properties was preferred.
Another cooling practice utilized insulating disks poised above the axle hub to reduce the dissipation of radiant heat from the hub area of the wheel. The wheels moved in intermittent or discrete steps to perform the cooling cycle. This practice and the associated insulating disks, which are applied after the drawing furnace, are taught in U.S. Pat. No. 3,753,789 to Kucera et al.
No known assembly or method has provided controlled cooling of the as-cast railroad wheels with discrete or individual parametric control to provide the requisite wheel temperature for subsequent manufacturing practices.